Researchers at BitDefender have discovered a new type of malicious software that collects passwords for banking sites but targets only Firefox users.

The malware, which BitDefender dubbed “Trojan.PWS.ChromeInject.A” sits in Firefox’s add-ons folder. The malware runs when Firefox is started.

The malware uses JavaScript to identify more than 100 financial and money transfer Web sites, including Barclays, Wachovia, Bank of America, and PayPal along with two dozen or so Italian and Spanish banks. When it recognizes a Web site, it will collect logins and passwords, forwarding that information to a server in Russia.

Users could be infected with the Trojan either from a drive-by download, which can infect a PC by exploiting a vulnerability in a browser, or by being duped into downloading it. When it runs on a PC, it registers itself in Firefox’s system files as “Greasemonkey,” a well-known collection of scripts that add extra functionality to Web pages rendered by Firefox.